1. A stroller gate check bag
If you're traveling by plane and bringing a stroller, you are sometimes allowed to bring it with you to the gate and check it there, but sometimes it will have to be checked with the rest of your luggage. Often, it will then have to be put into a gigantic plastic bag and put with the special luggage. A stroller gate check bag can make it a lot easier to pack up your stroller, and it can also protect it (at least a bit) from the rough treatment it might receive in the luggage hold. There are lots of brands, including Peg Perego's Stroller Travel Bag and McLean's bag from Sky Mall. You can read more about strollers in my post Flying with kids: all about strollers.
2. A car seat travel accessory
I have only brought a car seat on a flight once, and found it to be something of a hassle (partly because one of the airlines we traveled with allowed it on board, and another didn't). However, when traveling with a baby or young toddler it can definitely make the airplane seat comfier for them. And this contraption will make it a lot easier to take the car seat with you on your trip. Lots of different brands are available, including GoGo Kidz Travelmate. You can read more about bringing child car seats on planes here.
3. A kid-sized travel mattress
When my kids were babies, I usually brought along a fleece blanket that I could spread on the floor or on a seat wherever we went for them to lay on and sleep on. A travel mattress would have been a good option too, especially since it could also be used at your destination, if you're staying in a hotel or with friends or relatives. One option is something like the PeaPod Plus Baby Travel Bed. Another option that looks really good for traveling with is the kikko'pilo mini memory foam mattress.
4. A nursing cover
Breastfeeding is a very convenient way to feed your baby when you're traveling: no accessories to pack, no washing up (unless you count your nursing bra/nursing pads...), and no microwave needed. (I did the bottle-travel with my oldest who never learned to latch and the breastfeeding with my younger one: both ways work just fine on a plane, but yes, breastfeeding was easier.) A nursing cover is a great way to get some privacy for you and your baby when it's time to eat. There is a wide variety of products out there, including the Bamboobies Cute Little Nursing Cover and the Jolly Jumper Pashmama Nursing Cover.
5. Reusable snack containers
I'm a big believer in bringing snacks along on flights, and the best way to do that is in some kind of packable, crush-resistant, reusable containers. There are tons of options here, including the Zo-Li On-The-Go Travel Formula & Snack Dispenser, Lock & Lock containers and On-the-Go snack cups.
6. A crazy kids' book
I'm sneaking this in here, because a good book can be a great travel accessory on board, at the airport, or at bedtime. My 4 year old daughter's current favorite is the very strange, zany, and funny The Magic Mustache by Gary Barwin. It's a different take on Jack and the beanstalk, and involves a nose (he's the main character), his parents the eyeballs, a magic moustache that grows into a bushy beard, and a very ferocious, giant mouth... It sounds nuts, and it is, but my daughter is smitten. Right now, it doesn't look like the book is readily available from Amazon, but look for it at your local library! Or you can get it from the author, Gary Barwin, himself! (You can read his own comment below this post! His email is: firstname.lastname@example.org, and his website is found here. It's definitely a unique book, and well worth checking out!)
7. A reusable bag for liquids
These days of course, all liquids you bring in your hand luggage on a plane have to be put into a see-through plastic bag. I love ziploc bags, but yes, I have considered getting a reusable bag for this purpose. Travelon has a TSA Compliant Travel Bag with Bottles that is nicely priced, and looks very convenient. You can read more about the rules and regulations for air-travel with liquids in my post Flying with liquids.
8. Comfy travel pillows
No matter what kind of pillows you bring on board, it is difficult for adults and kids to sleep comfortably in economy class. But a travel pillow can definitely help. There are inflatable ones (I like these because they're easy to pack), and others made of memory foam or other materials. The dolphin-shaped Lil' Lewis Explorers Dolphin Neck Pillow looks really cute for example. Another idea I hadn't seen before is the Skyrest travel pillow for kids. This product is inflatable and fills the space between two seats on the plane, making a nice, comfy footrest for your child. (I've sometimes resorted to stuffing a carry-on on the floor for them to rest their feet on, but this looks a little nicer.)
9. A tablet or e-reader
A tablet or e-reader can be a very good thing if you're stuck at the airport, or have to entertain your kids on a long flight. Load your e-reader up with books, books, and more books. If you have a tablet, load it up with entertaining games and apps (remember that you won't necessarily be able to stay online everywhere, so make sure you have some offline content you can use). If you do have an internet connection, a tablet could allow you to watch streaming content like movies, TV-shows, or "just" YouTube videos. Watching YouTube in a hotel room when there is nothing on TV in a language your kids can understand can be a sanity-saver!
10. ID Wristbands
This is one of those items you hope you don't really need, but maybe it would be a good idea to get, just in case. I was thinking it would be especially handy if you're going to somewhere big and busy, like Disneyland or other amusement parks or resorts. You simply write down your contact information on a piece of paper that is inside the band, and then your child just wears it (and you hope it will never, ever have to looked at by other people...). There are lots of different brands, including the Infoband Child Safety ID Wristband and the Velcro Child ID Bracelet.